Exhaust-gas energy recovery system and method for exhaust-gas energy recovery

ABSTRACT

The invention relates to an exhaust-gas energy recovery system, comprising an exhaust-gas line system (111) for conducting exhaust gases of an internal combustion engine and comprising a motor-generator device (101), which can be driven by means of exhaust-gas energy in order to produce electric current. The exhaust-gas line system (111) comprises a first line arm (124) to the motor-generator device (101) for conducting exhaust gases into the motor-generator device (101). The motor-generator device comprises a motor (100), which is arranged in such a way that the motor can be driven by a pressure of exhaust gas flowing through the motor. The invention further relates to a corresponding method for exhaust-gas energy recovery.

The present invention relates to an exhaust gas energy recovery system according to the preamble to claim 1, and to a method for exhaust gas energy recovery according to the preamble to claim 13.

For the goal of efficient operation of combustion engines, in particular in vehicles, efforts are made to improve characteristics of the combustion process. On the other hand, the total efficiency can also be increased by further use of the energy of exhaust gases produced by the combustion process.

To this end, a generic exhaust gas energy recovery system comprises an exhaust line system for guiding exhaust gases of a combustion engine, and a motor/generator unit which can be driven by exhaust energy to generate electrical energy.

Similarly, in a generic method for exhaust gas energy recovery, electrical energy is generated by a motor/generator unit using exhaust gas energy.

Conventional exhaust gas energy recovery systems use exhaust waste heat to generate electrical energy. To this end, heat of the exhaust gas is transferred to another working fluid. The working fluid is usually arranged in a closed circuit and passes the motor. A line system of the working fluid comprises further components, for example a pump, a condenser, facultatively further heaters and a recuperator. The selection of these components depends on the instant thermodynamic cycle. In particular, Rankine cycles are used, which are also referred to as ORC or Organic Rankine Cycles.

Conventional exhaust gas energy recovery systems may be able to make use of a substantial part of the exhaust gas energy. However, the subsequently emitted exhaust gas still has much energy that remains unused.

It may be regarded an object of the invention to provide an exhaust gas energy recovery system and a method for exhaust gas energy recovery which use exhaust gas energy particularly efficiently.

This object is solved with the exhaust energy recovery system comprising the features of claim 1, and the method comprising the features of claim 13.

Preferred variants of the exhaust energy recovery system of the invention of the method of the invention are subject-matter of the dependent claims and are also illustrated in the following description.

In the exhaust gas energy recovery system of the above-referenced kind, according to the invention the exhaust line system comprises a line (which is referred to in the following as a first line arm) to the motor/generator unit for guiding exhaust gases into the motor/generator unit, and the motor/generator unit comprises a motor which is arranged to be driven by a pressure of passing exhaust gases.

Similarly, according to the invention the method of the above-referenced kind comprises: guiding exhaust gases through a line of an exhaust line system into the motor/generator unit which comprises a motor that is driven by a pressure of exhaust gases flowing through the motor.

As an important idea of the invention, the pressure of the exhaust gases is used to generate electrical energy. Whereas hitherto only the heat of the exhaust gases has been used for generating electrical energy, the invention allows to additionally or alternatively use the exhaust gas pressure. Thus, kinetic energy of the exhaust gases, which has hitherto not been used, is at least partially converted into electrical energy.

As another central idea, exhaust gases are guided directly through the motor of the motor/generator unit. This is relevant for harvesting a particularly large share of the energy present in the pressure. Preferred designs of the motor for providing a particularly high efficiency also at rather low pressures, will be described later in more detail.

A motor/generator unit may generally indicate a device comprising a motor (engine) and being configured to convert motion energy into electrical energy. The necessary components may form a local unit or may be distributed at different spatial locations and being functionally connected. In particular, a generator may be provided which may in general be any device with which the (rotational) energy provided by the motor can be converted into electrical energy.

Also a combustion engine and an exhaust gas treatment system for cleaning exhaust gases may be regarded as part of the exhaust gas energy recovery system of the invention. The exhaust line system may be configured to guide at least a part of the exhaust gases of the combustion engine first through the motor of the motor/generator unit and then to the exhaust gas aftertreatment system. By arranging the motor/generator unit in the direction of flow upstream of the exhaust gas aftertreatment system, the energy present in the pressure may be used before the pressure through the exhaust gas aftertreatment system drops. If a turbocharger is provided, the motor/generator unit is preferably in the direction of flow downstream of the turbocharger. This facilitates in particular retrofitting conventional systems because the components up to the turbocharger, which components interact with each other, may remain unchanged.

The exhaust line system may comprise a fork/diversion from which a first line arm runs via the motor in the direction of the exhaust gas treatment system, and a second line arm bypasses the motor and runs in the direction of the exhaust gas treatment system. A control device may be provided at the fork and may be configured to set proportions in which the exhaust gas is divided to the first and second line arms. Advantageously, this control may in particular set a desired minimal pressure of the exhaust gas downstream of the engine/motor. Furthermore, a maximum amount of exhaust gases to the motor may be set to comply with limits of suitable operating conditions for the engine. Furthermore, this control adjusts the amount of generated electrical energy. For example, the amount of exhaust gases to the motor may be reduced if less electrical energy is required and/or if a storage for electrical energy is completely charged.

The control device may preferably comprise a rotatable flap or shutter. A rotation position of the flap/shutter defines the shares in which exhaust gas is guided into an entrance opening of the first line arm and into an entrance opening of the second line arm. In this way, a control can be effected reliably with simple means.

In principle, the motor of the motor/generator unit may be of any kind. However, it has to be considered that many motors only have a sufficient efficiency at rather high pressures. Furthermore, many motors impose narrow criteria for the passing medium, for example with regard to its temperature or viscosity. For a particularly high efficiency, the medium that flows through the motor should be the exhaust gas itself. In the following, preferred features of the motor are described for using the exhaust pressure particularly efficiently.

The engine is preferably a rotary engine in which at least one rotary plunger is rotated by passing exhaust gas and thus a drive shaft is rotated which drives the generator.

The rotary engine may comprise a housing which forms an inner room. At least one, preferably at least two or exactly two, rotary pistons may be arranged in the inner room. Furthermore, an inlet opening is provided for introducing exhaust gas into the inner room. The inlet opening is connected with the exhaust line system, in particular with the first line arm. The rotary engine further comprises an outlet opening for the exhaust gas, the outlet opening being arranged at the inner room at a side opposite to the inlet opening. The exhaust gas thus flow through the inner room and thus make the rotary pistons rotate. Each rotary piston preferably comprises at its outer circumference at least two sealing strips and at least two recesses, wherein the shapes of the recesses and of the sealing strips are chosen for engaging, in particular sealingly engaging, of the sealing strips of one rotary piston with the recesses of the other rotary piston, respectively. Furthermore, the sealing strips are radially sized for sealingly contacting a housing inner surface. A radial direction refers to the radius of the corresponding rotary piston, and thus the radial direction is transverse or perpendicular to the rotary direction of the respective rotary piston. The exhaust gas coming from the inlet opening pushes against (at least) some of the sealing strips, thus pushing these sealing strips against the housing inner surface. In particular, depending on a rotary position, at least one (or exactly one) of the sealing strips of each rotary piston may be exposed to incoming exhaust gas and may thus be pushed against the housing inner surface.

It may be regarded as an important characteristic of the invention to provide for a sealing of a rotary engine by means of sealing strips which are attached to or inserted at the rotary pistons. An exhaust gas pressure acts on the sealing strips and pushes these against the inner surface of the housing, which produces a particularly good sealing. The exhaust gas pressure thus leads to a certain deformation of the sealing strips which is important for an efficient sealing.

Such a deformation would not or hardly be possible if the whole outer circumference of a rotary piston were formed rigidly, in particular from the same material.

The sealing exhaust gas pressure may already be reached at a comparably low pressure. Furthermore, the viscosity of the exhaust gas only plays a minor role. The rotary engine of the invention may thus be deployed for many different exhaust gas compositions and under very different pressures. As a further advantage, lubricants or lubricating oils are not required with the rotary engine of the invention.

A particularly good sealing may be achieved if the sealing strips comprise a deformable or elastic material so that the sealing strips may be pushed/deformed by the exhaust gas against the housing inner surface. The material of the sealing strips is easier deformable or more elastic than a material of the rotary piston surrounding the sealing strips, in particular easier deformable or more elastic than the material in which the slots for receiving the sealing strips are formed, which slots are described in more detail further below.

The rotary pistons are sized and positions in the inner room such that the exhaust gas can only flow from the inlet opening to the outlet opening if the rotary pistons are thereby rotated. In other words, the two rotary pistons provide for a sealing at standstill such that no exhaust gas can flow through the inner room without rotation. For this sealing, a contact of the two rotary pistons is necessary. This contact provides that little or no exhaust gas may pass through the two rotary pistons. On the other hand, also a contact of the two rotary pistons to the housing inner surface is necessary for said sealing. This contact is provided for at least at a side facing outwards of the respective rotary piston, which is opposite the contact area between the rotary pistons. For example, by means of its sealing strips, each rotary piston may provide for a sealing contact with a neighbouring housing inner surface through an angle range of at least 150°, preferably at least 180° and particularly preferably more than 180°.

The sealing strips may extend in a longitudinal direction which is substantially parallel to the rotation axes of the two rotary pistons. In particular, an angle between the longitudinal direction and the rotation axes may be smaller than 20°, preferably smaller than 10°.

The two rotation axes of the two rotary pistons may be parallel to each other or at an angle which is not more than 40° or preferably not more than 20°. Furthermore, the two rotary pistons may be formed identically. If asymmetric sealing strips are used, as described further below, the rotary pistons may be identical to each other except for a mirror-inverted design or shape.

A rotary piston may be understood as a component that is rotatably mounted and rotates a driveshaft when it rotates. The rotation of the driveshaft may then be used to rotate other components, for example, and/or to drive a generator for generating electrical energy.

For attaching the sealing strips at the rotary pistons, the sealing strips may be accommodated in slots, i.e., grooves or similar recesses, formed at the respective outer circumference of the rotary pistons. In particular, the slots may be formed in the gear rims of the rotary pistons which are described in more detail further below. The sealing strips may be attached in the slots in basically any manner. The sealing strips may thus be exchangeable, thus allowing easy replacement of the sealing strips when necessary because of wear due to the sealing contact; without the necessity to replace further components of the rotary engine.

In a preferred variant, the sealing strips are formed as slot nuts for securely engaging with the slots in the rotary piston. This may be understood such that the sealing strips comprise a widening or a collar at their respective inner end which is received in the corresponding rotary piston. The slots which receive the sealing strips are formed such that said widening or collar securely engages with the slot.

In particular, the slots may be formed as T-slots and each of the slot nuts may comprise a laterally protruding collar for engagement with one of the T-slots. In a cross-section traverse or perpendicular to the rotation axis of the corresponding rotary piston, the slots may have the shape of a T. An end of the slot nuts facing the inner side of the rotary piston also has a T-shape such that the slot nuts are secured in the T-slot. In principle, threaded fasteners or adhesive attachments may also be provided for securing the sealing strips in the slots.

More generally but in particular in the above examples, the sealing strips and the corresponding slots may be formed such that the sealing strips are secured, i.e., cannot be moved, in a radial direction of the corresponding rotary piston. In contrast, in a perpendicular direction hereto, for example, in particular in the direction of the rotation axis of the rotary piston, a movement (and thus insertion and replacement) of the sealing strips may be possible. It is thus easily possible to replace worn or used sealing strips.

The sealing effect between the sealing strips and the housing inner surface depends on the deformation of the sealing strips. It may be preferable if the exhaust gas pressure causes a deformation of the sealing strips towards the housing inner surface and not a deformation of the sealing strips away from the housing inner surface. Each of the sealing strips has a surface which faces incoming exhaust gas, for a rotation angle position of the rotary piston at which the sealing strip contacts the housing inner surface. In the following, this surface is referred to as the exhaust gas contact surface or as the surface facing incoming exhaust gases. For providing a deformation for sealingly contacting the housing inner surface, the exhaust gas contact surface may preferably not have a convex shape or at least may not have a convex shape at its end facing the housing inner surface. It may be preferable that the exhaust gas contact surface may rather have a concave shape or at least may have a concave shape at its end facing the housing inner surface. Alternatively, also a substantially plane extension of the exhaust gas contact surface may provide a sufficient deformation, depending on the circumstances.

Each sealing strip has a rear side opposite the exhaust gas contact surface. This rear side does not face incoming exhaust gas when a rotation angle position of the rotary piston is such that the sealing strip contacts the housing inner surface or is next to the housing inner surface. The shape of the rear side also has consequences on the deformation and thus sealing effect. It may be preferable that the rear side is not concave or at least not concave at an end facing the housing inner surface. It may be preferable that the rear side is convex or has a convex end facing the housing inner surface. A sufficient sealing effect may also be possible with a linear or plane shape of the rear side.

The sealing strips may comprise an edge at which a sealing contact to the housing inner surface is achieved. An edge may result from a cross-section that is not rounded, in particular when the exhaust gas contact surface is concave or the rear side is convex.

It may be preferable that each rotary piston comprises (in particular exactly) two sealing strips at opposite angle positions at its respective outer circumference. In particular, the two angle positions may be offset to each other by a rotation angle of 180° about the rotation axis of the corresponding rotary piston. Furthermore, each rotary piston may comprise two recesses which are located at the outer circumference at angle positions that are also offset to each other by 180°, and are preferably offset to the angle positions of the two sealing strips by 90°. This has the effect that incoming exhaust gas always pushes against one of the sealing strips at each rotary piston and thus causes rotation of the rotary piston. Furthermore this design has the effect that a sealing of the two rotary pistons to the housing inner surface is provided independently from a current rotation position of the rotary piston.

The sealing strips may be sized and a housing inner surface may be formed such that the sealing strips sealingly contact the housing inner surface within a rotation angle range of the rotary piston. This rotation angle range may be opposite to a contact area between the two rotary pistons. Depending on the rotary position of the rotary piston, at least one of the sealing strips thus contacts the housing inner surface. It may be preferable that the shape of the housing inner surface is such that two sealing strips instead of just one sealing strip contact the housing inner surface over a rotary angle range, which may be between 5° and 20°, for example. Such an overlap ensures for all rotary positions that no exhaust gas may pass the rotary pistons without rotating the rotary pistons.

Each rotary piston may comprise a gear rim at its outer circumference. The rotary pistons may be arranged such that its gear rims intermesh. This substantially prevents exhaust gas from passing between the two rotary pistons. The exhaust gas is rather guided at the edge/perimeter between the rotary pistons and the housing inner surface.

The gear rims may be interrupted or broken by the recesses and the sealing strips, and otherwise may extend over the whole circumference of the two rotary pistons. A gear rim may be understood such that an outer circumferential surface of the corresponding rotary piston comprises radially protruding teeth. It may be preferable that each tooth extends over the whole height of the rotary pistons along their rotary axes.

In particular temperature variations may slightly change the relative position between the two rotary pistons. The intermeshing/engagement of the gear rims may, however, provide a sealing effect also with such positional variations. In contrast, the gear rims would be unsuited to provide a sealing towards the housing inner surface. Here no intermeshing teeth are provided and thus positional variations would lead to leakage currents. To avoid this, a sealing to the housing inner surface is not provided with the gear rims but with the sealing strips.

Depending on a rotary position of the two rotary pistons, a substantially sealing contact between the rotary pistons is provided either by the intermeshing gear rims or by one of the sealing strips of one rotary piston which protrudes into one of the recesses of the other rotary piston.

In a radial direction, the sealing strips may protrude further outwards from the respective rotary piston than the respective gear rim. The gear rim is thus always spaced apart from the housing inner surface. A free space is thus formed in between, through which exhaust gas is passed in the direction of the outlet opening. The free space is limited in the circumferential direction of the rotary pistons by the sealing strips.

The sealing strips protrude over the respective gear rim preferably by a radial distance which is between 5% and 30%, in particular between 10% and 25%, of a radius of the gear rim. This radius may be defined starting at the center point of the rotary piston to the outer circumference of the respective gear rim. The protruding radial distance affects the amount of a deformation of the sealing strip and thus affects sealing properties. Furthermore, the protruding radial distance is decisive for the amount of exhaust gas that is led along/past the corresponding rotary piston. It has become evident that with the above-mentioned values a good sealing effect can be achieved und a high efficiency can be achieved over a comparably large span of flow rate amounts.

A radial size of teeth of the gear rim is preferably not more than 15%, preferably not more than 10%, of a radius of the gear rim. In this way a exhaust gas flow between the two gear rims is sufficiently reduced. Larger teeth may have, depending on the exhaust gas, negative impacts of the exhaust gas flow. The radius of the gear rim may be defined by the distance from its center point to its outer circumference, i.e., to the outermost end of the teeth.

The exhaust gas energy recovery system of the invention may in general also be configured to use the heat energy of the exhaust gas in addition to the pressure of the exhaust gas to generate electrical energy. For using the heat energy it may be preferred to use an additional rotary engine which is designed as described herein or to use the same rotary engine which is also used for the exhaust gas pressure. If another rotary plunger (engine) is provided, it can be arranged in a working fluid circuit in which a working medium different from the exhaust gas circulates. Heat can be transferred from the exhaust gas to the working fluid in the working fluid circuit through a heat exchanger. The working fluid may in principle be of any kind. The working fluid circuit is designed as a thermodynamic cycle and comprises means for converting heat energy of the exhaust gas into motion energy. For example, the working fluid circuit may be designed as an organic Rankine cycle (ORC) and may comprise the components required for this. The rotary engine according to the invention is provided as the engine of the thermodynamic cycle (or: as the turbine that is used instead in such cycles). The passing-through exhaust gas is relaxed in that engine and rotation of the rotary pistons is thus caused. Instead of an ORC process, also other thermodynamic cycles may be used, in which cycles an engine is driven by heat energy. The thermodynamic cycle may for example comprise a feed pump, a heater or the heat exchanger, the rotary engine of the invention and a condenser as well as optionally a recuperator. The invention also relates to a vehicle, for example a passenger car or a truck comprising an internal combustion engine, wherein the vehicle comprises the exhaust gas energy recovery system according to the invention.

The rotary engine is described with two rotary pistons. In general, however, also further rotary pistons may be provided in the same inner room or in another inner room. Furthermore, the number of sealing strips and corresponding recesses may deviate from the numbers indicated with respect to the different embodiments.

The characteristics of the invention described as additional device features shall also be understood as variants of the method of the invention, and vice versa.

Further features and advantages of the invention are described below with reference to the attached schematic figures in which:

FIG. 1 is a schematic view of an embodiment of an exhaust gas energy recovery system of the invention;

FIG. 1 is a cross-section of a rotary engine of the exhaust gas energy recovery system of FIG. 1; and

FIG. 3 is an enlarged detail of FIG. 2.

Similar components and components with similar effects are generally indicated with the same reference signs throughout the figures.

FIG. 1 shows a schematic view of an embodiment of an exhaust gas energy recovery system 200 according to the invention. The system serves for generating electrical energy from energy from the exhaust gases emitted by a combustion machine (not depicted). The combustion machine may in particular be an internal combustion engine of a vehicle, however the invention is not limited to this.

The exhaust gas energy recovery system 200 comprises as important components a motor/generator unit 101 and an exhaust line system 111 which is configured to guide exhaust gases through an engine 100 of the motor/generator unit 101 and to drive the engine 100 in this way. The exhaust gas pressure is thus used to drive the engine 100. Advantageously, the exhaust energy provided by the pressure can be used for electricity generation.

The exhaust line system 111 comprises a line 110 which transports exhaust gases from a combustion engine which is not shown here. Further components, in particular a turbocharger, may be arranged between the combustion engine and the depicted line.

The line 110 leads to a fork at which the exhaust gas flowing through line 110 are guided into a first line arm 124 and/or a second line arm 122. This is controlled by a control device arranged at the fork, e.g., by a valve or shutter. The shutter is rotatably mounted wherein its rotation position defines in which proportions the exhaust gases are guided into the two line arms 122 and 124. While line arm 124 guides exhaust gas through the engine 100, the other line arm 122 bypasses the engine 100. Subsequently both lines 122 and 124 are united. The exhaust gas may then be further transported and processed in generally known ways. For example, it may be transported through an exhaust gas processing system 130 which serves for cleaning or filtering the exhaust gases.

The engine/generator unit 105 comprises, in addition to the engine 100, also a generator 105 which generates electrical energy by means of the rotational energy provided by the engine 100. To this end, the generator 105 may in particular be arranged on the shaft of the engine 100.

The electrical current output by the generator 105 may, for example, be transported to the battery of any consumers. If the exhaust gas energy recovery system is part of a vehicle, the consumers may be any components of the vehicle. Furthermore, a storage (not depicted in FIG. 1) may be provided in addition to the vehicle battery and loaded by the electrical current. This storage may for example be a Lithium ion battery or a (super) capacitor.

The engine 100 may be of any kind, however, it must be suitable for being driven by exhaust gases. Furthermore, it should have a particularly high efficiency at rather low exhaust gas pressures. This is achieved with an engine 100 which is described below with reference to FIGS. 2 and 3.

FIG. 2 shows schematically a cross-section of an embodiment of an engine 100 which is formed as a rotary engine 100. An enlarged detail thereof is shown in FIG. 3.

The rotary engine 100 comprises as important components two rotary pistons 20 and 30 which are arranged in an inner room 11 which is limited by a housing inner surface 12 of a housing 10.

An inlet opening 13 which is not shown in more detail allows exhaust gas to enter the inner room 11. The inlet opening 13 is connected with the first line arm of FIG. 1. An outlet opening 15 is furthermore provided at the inner room 11. If the exhaust gas flows from the inlet opening 13 through the inner room 11 to the outlet opening 15, it must pass both rotary pistons 20, 30, and has to rotate these. The reference signs 21 and 31 mark the rotation axes of the two rotary pistons 20 and 30. The rotation axes 21, 31 extend into the drawing plane.

The design of the rotary pistons 20, 30 is decisive for an efficient functioning. The rotary pistons shall provide a sealing to each other and a sealing to the surrounding housing inner surface 12 so that the exhaust gas cannot reach the outlet opening 15 if the rotary pistons 20, 30 do not move.

Simultaneously, the rotary pistons 20, 30 should be easily rotated by the exhaust gas, i.e., the rotary pistons 20, 30 should already rotate at low pressure.

To this end, the two rotary pistons 20 and 30 comprise sealing strips 25, 26, 35, 36 at their respective outer surfaces. The outer surfaces may be regarded as the shell surfaces of substantially cylindrical rotary pistons 20, 30. The sealing strips 25, 26, 35, 36 extend preferably over the whole height of the inner room 11, wherein the height may refer to a direction of the rotation axes 21, 31.

The rotary piston 20 comprises at least two, preferably exactly two, sealing strips 25, 26. Similarly, the rotary piston 30 comprises at least two, preferably exactly two, sealing strips 35, 36. The sealing strips 25, 26, 35, 36 extend radially beyond the remaining outer circumference of the respective rotary piston 20, 30. The sealing strips 25, 26, 35, 36 are preferably received in slots at the respective rotary piston 20, 30, and may preferably consist of a material different to the part of the rotary piston 20, 30 in which the slots are formed. The sealing strips 25, 26, 35, 36 may consist, in particular, of a deformable material, which may be, for example, rubber, resin or a plastic material. In this way the sealing strips 25, 26, 35, 36 may be slightly deformed by exhaust gas flowing against it, and may thus be pressed against the housing inner surface 12. In this way a particularly good sealing to the housing inner surface 12 is achieved. In principle, the sealing strips 25, 26, 35, 36 may also consist of a rigid material, for example a metal. Alternatively or additionally the sealing strips 25, 26, 35, 36 may be received with some leeway in their respective slots, and thus the exhaust gas pressure can slightly tilt the sealing strips 25, 26, 35, 36. In this way the sealing strips 25, 26, 35, 36 may in principle also be pressed against the housing inner surface 12.

The two rotary pistons 20, 30 are arranged in the inner room 11 such that they contact each other. In this way, a exhaust gas flow between the rotary pistons is substantially ruled out. The rotation axes 21 and 31 may be parallel to each other. However, also a tilt between the rotation axes 21, 31 is possible as long as a substantially sealing contact between the rotary pistons 20, 30 is ensured.

To this end, the rotary pistons 20, 30 also each comprise a gear rim 23, 33 at the respective outer circumference, which gear rim is rigidly connected with the remainder of the corresponding rotary piston 20, 30. The two gear rims 23, 33 are sized and arranged to intermesh. Thereby the two gear rims 23, 33 rotate jointly and form hardly any free spaces between each other. It is thus hardly possible for exhaust gas to flow between the two gear rims 23, 33.

Furthermore, the rotary pistons 20 and 30 comprise recesses 27, 28 and 37, 38, respectively, at their respective outer circumference. The number of recesses 27, 28 of the first rotary piston 20 is equal to the number of sealing strips 35, 36 of the second rotary piston 30. Similarly, the number of recesses 37, 38 of the second rotary piston 30 is equal to the number of sealing strips 25, 26 of the first rotary piston 20. Furthermore the recesses 27, 28, 37, 38 and the sealing strips 25, 26, 35, 36 are arranged at the two rotary pistons 20, 30 such that the sealing strips 25, 26 of the first rotary piston 20 mate with the recesses 37, 38 of the second rotary piston 30 when the two rotary pistons 20 and 30 rotate. Similarly the sealing strips 35, 36 of the second rotary piston 30 mate with the recesses 27, 28 of the first rotary piston 20. To this end, a recess and a sealing strip may alternate in 90° separations at the outer circumference of each rotary piston 20, 30, for example. In other words, the two sealing strips 25, 26 are distanced from each other by an azimuth angle of 180° (i.e., an angle of 180° about the rotation axes 21). Also the two recesses 27, 28 are separated from each other by an azimuth angle of 180°, and by an azimuth angle of 90° relative to the sealing strips 25, 26. This is analogously valid for the sealing strips 35, 36 and recesses 37, 38 of the other rotary piston 30. In general, also other angles are possible. Other azimuth angles result in particular if there are more than two sealing strips and two recesses per rotary piston 20, 30. Size and shape of the recesses are thus chosen such that the sealing strips may be received therein, in particular in a sealing manner.

Similar to the gear rims 23, 33, also the sealing strips 25, 26 35, 36 provide together with the recesses 27, 28, 37, 38 that exhaust gas can hardly pass between the two rotary pistons.

Independent from a current rotary position, always one of the sealing strips 25, 26 35, 36 of each rotary piston 20, 30 shall provide a sealing to the housing inner surface 12. To this end a rotation angle is relevant over which one and the same sealing strip 25, 26 35, 36 causes a sealing to the housing inner surface 12. This rotation angle may be larger than 180°, as shown in FIG. 2, and may for example be between 185° and 240°. To this end, the housing wall 12 may have the shape of a segment of a circle at each rotary piston, wherein this shape forms a segment of a circle which is larger than 180° and thus forms more than a semi-circle.

FIG. 3 shows in greater detail the reception of the sealing strips 25, 26 35, 36 in their corresponding slots. The sealing strip 35 is shown in its cross-section as an example for all sealing strips 25, 26 35, 36. The sealing strip 35 may have the shape of a profile, i.e., it may have the same cross-section throughout its length (in particular in the direction of the rotation axis 31). As shown, the cross-sectional shape forms a slot nut. Towards the inner end of the sealing strip, a collar 35C is formed, which engages with a T-shaped recess/slot. This inhibits that the slot nut may inadvertently come loose out of the slot of the rotary piston in a radial direction. Inserting and removing the slot nut 35 is possible in the longitudinal direction, i.e., in the direction of the rotation axis 31. By forming slot nuts, the sealing strips can be easily secured. Furthermore, also replacement is facilitated. This is relevant as gradual abrasion of the sealing strips 25, 26, 35, 36 may occur and may thus make a replacement necessary due to the sealing contact with the housing inner surface 12.

As shown in FIG. 2, the exhaust gas in the inner room 11 pushes against the rotary pistons 20, 30 and those sealing strips 25, 35 that face the inlet opening 13 in the momentary rotary position of the rotary pistons 20, 30. This pressure causes rotation of the rotary pistons 20, 30 in the direction of the arrows shown in FIG. 2.

For the rotation and in particular for the sealing effect of the sealing strips 25, 35, the shape of the sealing strips is important. This is explained in more detail with respect to FIG. 3, which shows a sealing strip 35 which protrudes radially from the gear rim 33. The sealing strip 35 has a point of maximal radial extension or an edge which extends into the paper plane (or in the direction of the rotation axis 31). Starting from this edge, the sealing strip 35 has a surface 35A or exhaust gas contact surface 35A facing the incoming exhaust gas (this is valid for rotation positions in which the sealing strip 35 contacts the housing inner surface 12). On the opposite side of said edge, the sealing strip 35 comprises another surface 35B which is referred to as a rear side 35B. The rear side 35B does not face the incoming exhaust gas when the sealing strip 35 contacts the housing inner surface 12.

The exhaust gas contact surface 35A comprises a recess or a concave shape, whereas the rear side 35B has an outwardly curved or convex shape. In this way, the outer end of the sealing strip 35, i.e., the radially furthest extending part, is deformed transversely or approximately perpendicularly to the radial direction by the exhaust gas flowing against it. The sealing strip 35 is thus pressed against the housing inner surface 12. In FIG. 3, the lower end of the sealing strip 35 is deformed approximately to the left and thus against the housing inner surface 12.

Advantageously, in this way a particularly good sealing is provided, without however causing unduly high friction between the sealing strips and the housing inner surface. Advantageously, already at a comparably low exhaust gas pressure, the rotary pistons may thus be set in rotation. Also exhaust gases at low pressure may thus be employed for energy usage.

The sealing strips may have other shapes than the described shapes. For example, it may thus suffice if the exhaust gas contact surface or the rear side is formed as described. The other side may, for example, be flat or shaped like the other side. It is also possible that the described shapes of the exhaust gas contact surface and the rear side are only formed at an end portion of the sealing strips and not across the whole part that radially protrudes beyond the corresponding gear rim. It may generally suffice for sufficient sealing properties if the sealing strips are deformable or movable relative to the gear rim and are, in particular, not formed integrally with the gear rim. As a central idea the exhaust gas pressure produced by a combustion engine can be used to generate electrical energy. This is possible with an engine which is preferably formed by the described rotary engine. 

1. An exhaust gas energy recovery system comprising: an exhaust line system (111) for guiding exhaust gases from a combustion engine, a motor/generator unit configured to be driven by exhaust energy to generate electrical energy, wherein the exhaust line system comprises a first line arm to the motor/generator unit for guiding exhaust gases to the motor/generator unit, wherein the motor/generator unit comprises a motor which is arranged to be driven by a pressure of passing exhaust gases, and wherein the motor is a rotary engine comprising: a housing defining an inner room, at least two rotary pistons arranged in the inner room, an inlet opening which is connected with the exhaust line system for introducing the exhaust gases into the inner room, and an outlet opening for the exhaust gas defined at the inner room at a side opposite to the inlet opening, wherein each rotary piston comprises a gear rim at its outer circumference, and the rotary pistons are arranged such that their gear rims mesh.
 2. The exhaust gas energy recovery system as defined in claim 1, further comprising: a combustion engine and an exhaust gas treatment system for cleaning exhaust gases, wherein the exhaust line system is configured for guiding at least a part of the exhaust gases of the combustion engine first through the motor of the motor/generator unit and then to the exhaust gas treatment system.
 3. The exhaust gas energy recovery system as defined in claim 1, wherein the exhaust line system comprises a fork from which a first line arm runs via the motor in the direction of the exhaust gas treatment system and a second line arm bypasses the motor; and runs in the direction of the exhaust gas treatment system, and wherein the exhaust gas energy recovery system further comprises a control device at the fork configured to set proportions in which the exhaust gas in divided to the first and second line arms.
 4. The exhaust gas energy recovery system as defined in claim 1, wherein the control device comprises a rotatable shutter, and wherein a rotation position of the rotatable shutter determines in which parts the exhaust gas in guided into the first line arm and the second line arm.
 5. The exhaust gas energy recovery system as defined in claim 1, further comprising: two rotary pistons, wherein each rotary piston comprises at least two sealing strips; and at least two recesses at its outer circumference, the shapes of the recesses; and the sealing strips are chosen for sealing engagement of the sealing strips of each one of the rotary pistons with the recesses of the respective other rotary piston, wherein the sealing strips are sized to sealingly contact a housing inner surface in a radial direction.
 6. The exhaust gas energy recovery system as defined in claim 5, wherein the sealing strips comprise a deformable material such that the sealing strips can be pressed against the housing inner surface by incoming exhaust gases.
 7. The exhaust gas energy recovery system as defined in claim 5, wherein each of the sealing strips comprises an exhaust gas contact surface facing inflowing exhaust gas when the respective rotary piston is at a rotation angle position at which said sealing strip contacts the housing inner surface (12), and wherein the exhaust gas contact surface has a concave shape.
 8. The exhaust gas energy recovery system as defined in claim 7, wherein each of the sealing strips has a rear side which is opposite the exhaust gas contact surface and which does not face inflowing exhaust gas when the respective rotary piston is at a rotation angle position in which said sealing strip contacts the housing inner surface, and wherein the rear side has a convex shape.
 9. The exhaust gas energy recovery system as defined in claim 5, wherein the rotary pistons comprise at their respective outer circumference slots for receiving and securing the sealing strips, and wherein the sealing strips are formed as slot nuts for securely coupling with the slots of the respective rotary piston.
 10. The exhaust gas energy recovery system as defined in claim 9, wherein the slots are formed as T-slots and each slot nut comprises a laterally protruding shroud for engaging with one of the T-slots.
 11. The exhaust gas energy recovery system as defined in claim 5, wherein each rotary piston comprises exactly two sealing strips at opposite angle positions at its outer circumference, and wherein each rotary piston comprises exactly two recesses arranged at the outer circumference at angle positions which are each offset by 90° relative to the angle positions of the two sealing strips.
 12. The exhaust gas energy recovery system as defined in claim 1, wherein the sealing strips protrude from their respective rotary piston further outwards in a radial direction than the respective gear rim.
 13. A method for exhaust gas energy recovery, the method comprising generating electrical energy from exhaust energy with a motor/generator unit, by: guiding exhaust gases to the motor/generator unit via a first line arm of an exhaust line system (111), wherein the motor/generator unit comprises a motor which is driven by a pressure of passing exhaust gases, wherein the motor is a rotary engine comprising: a housing defining an inner room, at least two rotary pistons arranged in the inner room, an inlet opening through which exhaust gases are transported from the exhaust line system into the inner room, and an outlet opening for the exhaust gas defined at the inner room at a side opposite to the inlet opening, each rotary piston comprises a gear rim at its outer circumference, and the rotary pistons are arranged such that their gear rims mesh. 